A 1-year study of chlorpyrifos manufacturing workers found a 13% lower mean plasma butyrocholinesterase activity than in a reference group (p<0.002), but no persistent symptoms of weakness or sensory deficits were reported (Albers et al., 2004). In comparison, male human volunteers taking 0.1 mg/kg/day orally for nine days showed a mean inhibition of plasma cholinesterase of about 65% (EPA).

Persistent sensory neuropathy with or without memory impairment was reported in cases of prolonged moderate exposure in enclosed sites fumigated with chlorpyrifos (Kaplan et al., 1993). Cases of deficits in grip strength measured seven weeks or more following hospitalization for chlorpyrifos poisoning have also been reported (Meggs, 2003; Miranda et al., 2002).